1. Field of the Invention
The present invention relates to a semiconductor light emitting device composed of a Group III-V nitride based semiconductor (hereinafter referred to as a nitride based semiconductor) such as BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride) or TlN (thallium nitride) or their mixed crystal.
2. Description of the Background Art
In recent years, nitride based semiconductor laser devices emitting blue or violet light have been investigated and developed as light sources for recording or reproduction used for high-density and large-capacity optical disk systems.
FIG. 8 is a schematic cross-sectional view showing an example of a conventional nitride based semiconductor laser device.
The semiconductor laser device shown in FIG. 8 is constructed by forming, on the C(0001) plane of a sapphire substrate 81, a buffer layer 82 composed of undoped AlGaN, an undoped GaN layer 83, an n-GaN contact layer 84 composed of n-GaN, a crack preventing layer 85 composed of n-InGaN, an n-AlGaN cladding layer 86 composed of n-AlGaN, a light emitting layer 87 composed of InGaN, a p-AlGaN cladding layer 91 composed of p-AlGaN, and a p-GaN contact layer 92 composed of p-GaN in this order by MOCVD (Metal Organic Chemical Vapor Deposition).
The light emitting layer 87 is constructed by stacking an n-GaN optical guide layer 88 composed of n-GaN, an MQW active layer 89 composed of InGaN and having a multi-quantum well (MQW) structure, and a p-GaN optical guide layer 90 composed of p-GaN in this order.
A portion to a predetermined depth of the p-AlGaN cladding layer 91 from the p-GaN contact layer 92 is etched away. Consequently, a striped ridge portion 93 comprising the p-GaN contact layer 92 and the p-AlGaN cladding layer 91 is formed, and a flat portion is formed in the p-AlGaN cladding layer 91. A p electrode 131 is formed on the p-GaN contact layer 92 in the ridge portion 93. Further, a partial region from the flat portion of the p-AlGaN cladding layer 91 to the n-GaN contact layer 84 is etched away, so that an n electrode forming region 94 in the n-GaN contact layer 84 is exposed. An n electrode 132 is formed on the exposed n electrode forming region 94.
An insulating film 95 composed of an Si oxide such as SiO2 is formed on both side surfaces of the ridge portion 93, an upper surface of the flat portion of the p-AlGaN cladding layer 91, a side surface from the p-AlGaN cladding layer 91 to the n-GaN contact layer 84, and an upper surface of the n-GaN contact layer 84, excluding a region where the n electrode 132 is formed.
In the semiconductor laser device shown in FIG. 8, the difference in the refractive index between the light emitting layer 87 and the n-AlGaN cladding layer 86 and the p-AlGaN cladding layer 91 is small, i.e., approximately one-forth to one-third that in the conventional AlGaAs based semiconductor laser device, for example. Therefore, light generated in the MQW active layer 89 in the light emitting layer 87 is not easily waveguided to the light emitting layer 87.
Furthermore, the semiconductor laser device has a so-called anti-waveguided structure in which the refractive indexes of the n-GaN contact layer 84 and the p-GaN contact layer 92 respectively positioned outside the n-AlGaN cladding layer 86 and the p-AlGaN cladding layer 91 for confining light generated in the MQW active layer 89 in the light emitting layer 87 are higher than those of the n-AlGaN cladding layer 86 and the p-AlGaN cladding layer 91. Accordingly, a fundamental vertical transverse mode is not easily obtained.
A contact layer composed of a material having a high absorption coefficient, for example, GaAs can absorb light which has spread out of the cladding layers. However, the p-GaN contact layer 92 composed of GaN, as described above, cannot absorb light which has spread out of the p-AlGaN cladding layer 91 because it has a low absorption coefficient.
In the semiconductor laser device shown in FIG. 8, therefore, it is difficult to sufficiently confine light in the light emitting layer 87, so that the vertical transverse mode is easy to be a high-order mode. Therefore, it is difficult to reduce a threshold current in the semiconductor laser device.
In order to prevent the vertical transverse mode from being the high-order mode, the Al composition ratios of the n-AlGaN cladding layer 86 and the p-AlGaN cladding layer 91 are increased (made larger than 0.07, for example; AlxGa1−xN, x>0.07 when expressed by a general formula) or Al is added at a composition ratio of several percent to the n-GaN contact layer 84 (Al is added at a composition ratio of approximately 0.02, for example; Al0.02Ga0.98N). Consequently, a fundamental transverse mode is easily obtained. However, the Al composition ratios are thus increased, so that a growth layer is easily cracked. As a result, the yield of the device is greatly lowered.
The MQW active layer 89 in the semiconductor laser device is composed of InGaN having a higher lattice constant than those of GaN and AlGaN. The crystallinity of the MQW active layer 89 composed of InGaN is degraded if the thickness thereof is increased. In order not to degrade the crystallinity of the MQW active layer 89, therefore, the thickness of the MQW active layer 9 is decreased to several tens of angstroms. When the thickness of the MQW active layer 89 is thus decreased, however, light is not easily confined in the light emitting layer 87, and the vertical transverse mode is further easy to be a high-order mode. Therefore, it is more difficult to reduce the threshold current in the semiconductor laser device.
On the other hand, the p-AlGaN cladding layer 91 composed of p-AlGaN is high in resistance. Accordingly, in the semiconductor laser device, a series resistance between electrodes is increased. Therefore, it is difficult to reduce an operating voltage in the semiconductor laser device. Particularly when the semiconductor laser device is operated at a low temperature of not more than 0° C., the resistance of the p-AlGaN cladding layer 91 is further increased. Accordingly, the operating voltage is further increased. Consequently, a device breakdown such as a dielectric breakdown easily occurs.